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Part I: Micromechanics of dense suspensions: microscopic interactions to macroscopic rheology & Part II: Motion in a stratified fluid: swimmers and anisotropic particlesRishabh More (8436243) 18 April 2022 (has links)
<p><b>Part I: Micromechanics of dense suspensions</b></p><p>Particulate suspensions are ubiquitous in the
industry & nature. Fresh concrete, uncured solid rocket
fuel, & biomass slurries are typical industrial applications, while milk & blood are examples of naturally occurring suspensions. These
suspensions exhibit many non-Newtonian properties like rate-dependent rheology &
normal stresses. Other than volume fraction, particle material, inter-particle interactions determine the rheological behavior of suspension. The average
inter-particle gaps between the neighboring particles decrease significantly as
the suspension volume fraction approaches the maximum packing fraction in dense
suspensions. So, in this regime, the short-ranged non-contact interactions are
important. In addition, the particles come into contact due to
asperities on their surfaces. The surface asperities are present even in the
case of so-called smooth particles, as particles in real suspensions are not
perfectly smooth. Hence, contact forces become one of the essential factors to determine the rheology of
suspensions.</p><p> </p><p>Part I of this thesis investigates the effects
of microscopic inter-particle interactions on the rheological properties of
dense suspensions of non-Brownian particles by employing discrete particle
simulations. We show that increasing the roughness size results in a rise in
the viscosity & normal stress difference in the suspensions.
Furthermore, we observe that the jamming volume fraction decreases with the
particle roughness. Consequently, for suspensions close to jamming,
increasing the asperity size reduces the critical shear rate for shear
thickening (ST) transition, resulting in an early onset of discontinuous ST
(DST, a sudden jump in the suspension viscosity) in terms of volume fraction, &
enhances the strength of the ST effect. These findings are in excellent
agreement with the recent experimental measurements & provide a deeper
understanding of the experimental findings. Finally, we propose a constitutive
model to quantify the effect of the roughness size on the rheology of dense ST
suspensions to span the entire phase-plane. Thus, the constitutive model and
the experimentally validated numerical framework proposed can guide
experiments, where the particle surface roughness is tuned for manipulating the
dense suspension rheology according to different applications. </p><p> </p><p>A typical dense non-Brownian particulate
suspension exhibits shear thinning (decreasing viscosity) at a low shear rate
followed by a Newtonian plateau (constant viscosity) at an intermediate shear
rate values which transition to ST (increasing viscosity) beyond a critical
shear rate value and finally, undergoes a second shear-thinning transition at
an extremely high shear rate values. This part unifies & quantitatively
reproduces all the disparate rate-dependent regimes & the corresponding
transitions for a dense non-Brownian suspension with increasing shear rate. The
inclusion of traditional hydrodynamic interactions, attractive/repulsive DLVO
(Derjaguin and Landau, Verwey and Overbeek), contact
interactions, & constant friction reproduce
the initial thinning as well as the ST transition. However, to
quantitatively capture the intermediate Newtonian plateau and the second thinning, an additional interaction of non-DLVO origin & a
decreasing coefficient of friction, respectively, are essential; thus,
providing the first explanation for the presence these regimes.
Expressions utilized for various interactions and friction are determined from
experimental measurements, resulting in an excellent quantitative agreement
with previous experiments. </p><p><br></p><p><b>Part II: Motion in a stratified fluid</b></p><p>Density variations due to temperature or
salinity greatly influence the dynamics of objects like particles, drops, and
microorganisms in oceans. Density stratification hampers the vertical flow &
substantially affects the sedimentation of an isolated object, the hydrodynamic
interactions between a pair, and the collective behavior of suspensions in
various ways depending on the relative magnitude of stratification inertia
(advection), and viscous (diffusion) effects. This part investigates these
effects and elicits the hydrodynamic mechanisms behind some commonly observed
fluid-particle transport phenomena in oceans, like aggregation in horizontal
layers. The physical understanding can help us better model these phenomena
and, hence, predict their geophysical, engineering, ecological, and
environmental implications. </p><p><br></p><p>We investigate the self-propulsion of an
inertial swimmer in a linear density stratified fluid using the archetypal
squirmer model, which self-propels by generating tangential surface waves. We
quantify swimming speeds for pushers (propelled from the rear) and pullers
(propelled from the front) by direct numerical solution. We find that
increasing stratification reduces the swimming speeds of swimmers relative to
their speeds in a homogeneous fluid while reducing their swimming efficiency.
The increase in the buoyancy force experienced by these squirmers due to the
trapping of lighter fluid in their respective recirculatory regions as they move
in the heavier fluid is one of the reasons for this reduction. Stratification
also stabilizes the flow around a puller, keeping it axisymmetric even at high
inertia, thus leading to otherwise absent stability in a homogeneous fluid. On
the contrary, a strong stratification leads to instability in the motion of
pushers by making the flow around them unsteady 3D, which is otherwise steady
axisymmetric in a homogeneous fluid. Data for the mixing efficiency generated
by individual squirmers explain the trends observed in the mixing produced by a
swarm of squirmers. </p><p><br></p><p>In addition, the ubiquitous vertical density
stratification in aquatic environments significantly alters the swimmer
interactions affecting their collective motion &consequently ecological and
environmental impact. To this end, we numerically investigate the interactions
between a pair of model swimming organisms with finite inertia in a linear
density stratified fluid. Depending on the squirmer inertia and stratification,
we observe that the squirmer interactions can be categorized as i) pullers
getting trapped in circular loops, ii) pullers escaping each other with
separating angle decreasing with increasing stratification, iii) pushers
sticking to each other after the collision and deflecting away from the
collision plane, iv) pushers escaping with an angle of separation increasing
with stratification. Stratification also increases the contact time for
squirmer pairs. The results presented can help understand the mechanisms behind
the accumulation of planktonic organisms in horizontal layers in a stratified
environment like oceans and lakes. </p><p><br></p><p>Much work has been done to understand the settling dynamics of spherical particles in a homogeneous and stratified fluid. However, the effects of shape anisotropy on the settling dynamics in a stratified fluid are not entirely understood. To this end, we perform numerical simulations for settling oblate and prolate spheroids in a stratified fluid. We find that both the oblate and prolate spheroids reorient to the edge-wise and partially edge-wise orientations, respectively, as they settle in a stratified fluid completely different from the steady-state broad-side on orientation observed in a homogeneous fluid. We observe that reorientation instabilities emerge when the velocity magnitude of the spheroids falls below a particular threshold. We also report the enhancement of the drag on the particle from stratification. The torque due to buoyancy effects tries to orient the spheroid in an edge-wise orientation, while the hydrodynamic torque tries to orient it to a broad-side orientation. The buoyancy torque dominates below the velocity threshold, resulting in reorientation instability.<br></p>
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Dual-Component Gelatinous Peptide/Reactive Oligomer Formulations as Conduit Material and Luminal Filler for Peripheral Nerve RegenerationKohn-Polster, Caroline, Bhatnagar, Divya, Woloszyn, Derek J., Richtmyer, Matthew, Starke, Annett, Springwald, Alexandra H., Franz, Sandra, Schulz-Siegmund, Michaela, Kaplan, Hilton M., Kohn, Joachim, Hacker, Michael C. 21 December 2023 (has links)
Toward the next generation of nerve guidance conduits (NGCs), novel biomaterials and
functionalization concepts are required to address clinical demands in peripheral nerve regeneration
(PNR). As a biological polymer with bioactive motifs, gelatinous peptides are promising building
blocks. In combination with an anhydride-containing oligomer, a dual-component hydrogel system
(cGEL) was established. First, hollow cGEL tubes were fabricated by a continuous dosing and
templating process. Conduits were characterized concerning their mechanical strength, in vitro
and in vivo degradation and biocompatibility. Second, cGEL was reformulated as injectable shear
thinning filler for established NGCs, here tyrosine-derived polycarbonate-based braided conduits.
Thereby, the formulation contained the small molecule LM11A-31. The biofunctionalized cGEL filler
was assessed regarding building block integration, mechanical properties, in vitro cytotoxicity, and
growth permissive effects on human adipose tissue-derived stem cells. A positive in vitro evaluation
motivated further application of the filler material in a sciatic nerve defect. Compared to the empty
conduit and pristine cGEL, the functionalization performed superior, though the autologous nerve
graft remains the gold standard. In conclusion, LM11A-31 functionalized cGEL filler with extracellular
matrix (ECM)-like characteristics and specific biochemical cues holds great potential to support PNR.
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The Hydrodynamic Interaction of Two Small Freely-moving Particles in a Couette Flow of a Yield Stress FluidFirouznia, Mohammadhossein January 2017 (has links)
No description available.
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Comportement de fluides complexes sous écoulement : approche expérimentale par résonance magnétique nucléaire et techniques optiques et simulations numériques / Behaviour of complex fluids flow : experimental study by nuclear magnetic resonance and optical techniques and numerical simulationRigal, Claire 23 May 2012 (has links)
Cette thèse est une contribution à la fois expérimentale, théorique et numérique à l'étude des écoulements bidimensionnels de fluides complexes dans une conduite cylindrique présentant des singularités et dans une géométrie annulaire à cylindres excentrés. Le fluide utilisé est une solution de xanthane à différentes concentrations présentant un caractère non newtonien rhéofluidifiant. L'objectif principal de cette thèse est la caractérisation de l'influence des propriétés rhéofluidifiantes sur le comportement des zones de recirculation, en terme de morphologie, de positionnement et d'intensité, par l'utilisation et le développement de techniques de mesures non intrusives et performantes. La première méthode expérimentale utilisée une technique laser classique: la vélocimétrie par images de particules. La seconde technique mise en oeuvre est une méthode originale: la vélocimétrie par imagerie par résonance magnétique. Elle est utilisée pour la première fois au laboratoire pour la mesure de champ de vitesse d'écoulement de fluides complexes en conduite cylindrique, représentant l'intérêt majeur de cette thèse. La première partie de notre travail consiste en une description rhéologique complète de nos fluides modèles avec la détermination de leur loi de comportement et la mise en évidence de leurs propriétés viscoélastiques, par ailleurs négligeables. Par la suite les mesures de champ de vitesse des écoulements bidimensionnels étudiés et la représentation des lignes de courant montrent que les propriétés rhéofluidifiantes influencent très fortement la structure et la morphologie de ces écoulements et le comportement des zones de recirculation. Par une étude fine nous observons qu'il existe une compétition entre les effets d'inertie et les effets rhéofluidifiants induisant un champ de contrainte variable qui modifie le positionnement et la taille de la zone de recirculation. Nous montrons également que l'augmentation du caractère rhéofluidifiant affaiblit son intensité de la zone de recirculation. Enfin, des simulations numériques utilisant la loi de comportement macroscopique déterminée par rhéométrie classique ont été réalisées avec le logiciel Fluent. Une bonne concordance est observée entre les résultats de ces simulations numériques et les expérimentaux. Cette comparaison permet ainsi de valider le code de calcul et la loi de comportement, utilisée pour les simulations numériques au travers de sa modélisation suivant la loi de Cross, pour les écoulements considérés / This thesis is an experimental and numerical study of structured fluids bidimensional flows in a cylindrical pipe with singularity and in an annular geometry with eccentric cylinders. The objective of this thesis is to characterize the influence of the shear thinning properties on the recirculation zones by using efficient and non-intrusive techniques: particle image velocimetry and velocimetry by nuclear magnetic resonance imaging. Materials are xanthane solutions at different concentrations. In the first part, we determine the rheological and viscoelastic properties of the fluids used. The second part concerns the measured velocity field. It is shown that the shear thinning behavior have a strongly influence on the structure and the morphology of these flows and the pattern of the recirculation zones. Simultaneously, numerical simulations performed by Fluent and using the rheological behavior. A good concordance is observed between the experimental and numerical results. For the flows considered here, this comparison allows to validate the computational code and the behavior law used in the numerical simulations and modelling by a Cross model
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Optimisation of methane production from anaerobically digested cow slurry using mixing regime and hydraulic retention timeHughes, Kevin Lewis William January 2015 (has links)
AD is regarded as a sustainable technology that could assist the UK Government meet internationally agreed GHG emission targets by 2050. However, the mature status of the technology is based on expensive systems that rely on high energy feedstock to be profitable. Meanwhile, the natural biodegradation of cow slurry is a recognised contributor to climate change despite having a relatively low CH4 potential because of the large volumes produced. Economic mixing is essential to the cost-effectiveness of farm AD but techniques applied are not always appropriate as slurry is a shear thinning thixotropic Herschel-Bulkley fluid and therefore challenging to mix. The apparent viscosity of slurry and the shear stress induced was most influenced by solids content (exponential change) followed by temperature (linear). Most shear thinning occurred before a rising shear rate of 20s-1 was achieved with the fluid acting near-Newtonian above. Thixotropic recovery occurred within 1 hour of resting. Rheological values were also much higher than previously reported. Highest CH4 production occurred in the first 10 days of the batch process using a range of mixing regimes with different shear rates and rest periods. During fed-batch operations, changing shear rate had a minimal effect on CH4 production using a 30-day HRT whereas shorter rest periods increased production. Specific CH4 production rate was highest when feeding and mixing coincided. However, when HRT was reduced (OLR increased) the CH4 produced by all mixed regimes significantly increased with highest values being achieved using high intensity mixing rested for short periods. Lower HRTs also requires smaller digesters. Parasitic mixing energy invariably had the most influence on net energy production. Signs of instability were evident after 20 days using the low HRT. Significant microbial adaptation was also observed as the experiments progressed. The research outcomes demonstrate that mixing regime and HRT can be managed to maximise net energy production whilst reducing capital expenditure.
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